A differential pressure transducer is a device that provides an output indicative of a difference between two input pressures. For example, when a first pressure P1 is applied to one face of a deflecting diaphragm and a second pressure P2 is applied to the other face of the deflecting diaphragm the resulting deflection of the diaphragm will be determined by the difference in pressure (as for example P1-P2). There are other differential pressure transducers in the prior art that essentially use two separate semiconductor structures. Each structure has its own diaphragm and the piezoresistors employed on the diaphragm surfaces are connected together to form a bridge which provides a differential output.
A pressure sensor of a differential pressure transducer is often enclosed inside an oil filled capsule to protect the sensor from harsh environments. One drawback of this encapsulation is that as the temperature changes, the oil expands and contracts and creates an additional pressure on the sensor. Thus, as the oil expands the metal isolation diaphragm must deflect slightly in order to make room for the extra volume of oil. This deflection causes the metal diaphragm to impart a small pressure (a “back pressure”) into the oil which is then transmitted to the sensor. This pressure is generally on the order of a few PSI for every 100° F. change in temperature but can vary a great deal depending on the volume of oil and the stiffness of the metal diaphragm. For most pressure sensors this additional pressure is relatively small compared to other errors and can be electrically corrected in a number of different ways.
However, when the sensor is a relatively low pressure sensing device, it is more difficult to correct these errors, and extra effort must be made to eliminate the cause of this undesirable back pressure. This is also true in high heat environments or with fluctuating temperature environments. These challenges generally occur within differential pressure sensors where both the main and reference ports of the transducer are oil-filled. One solution to this problem is to make the two oil filled pressure capsules identical in configuration such that the pressure on the front and back of the sensor cancel out (as disclosed in U.S. Pat. No. 7,743,662, herein incorporated by reference), which is not always possible due to geometric considerations. It is to this need that the present invention is directed.